1. Field of the Invention
The present invention relates to a technique for forming print data utilized by an ink jet printer. The ink jet printer of the present specification includes all devices for printing onto a print medium by means of discharging ink (printers, copiers, fax machines, multifunctional products, etc.).
2. Description of the Related Art
Ink jet printers print onto a print medium by means of discharging ink The manner in which printing is performed by an ink jet printer will be described with reference to FIG. 18. An ink jet printer 151 has an ink jet head 152 that moves with respect to a print medium 150. In FIG. 18, the ink jet head 152 moves in a Y direction with respect to the print medium 150. The ink jet head 152 passes a front side of the print medium 150. The ink jet head 152 has a plurality of nozzles 153˜157. The nozzles 153˜157 are aligned in an X direction that is orthogonal to the Y direction. The nozzles 153˜157 can discharge ink droplets in the direction perpendicular to the page.
The ink droplets are discharged from the nozzles 153˜157 while the ink jet head 152 is moving with respect to the print medium 150. One dot is formed on the print medium 150 by discharging one or a plurality of ink droplets from one nozzle. In FIG. 18, 35 dots have been formed on the print medium 150. The dots aligned in the Y direction have been formed by one nozzle. For example, a dot line D1 has been formed by continuously discharging ink droplets from the nozzle 153. Similarly, a dot line D2 has been formed by the nozzle 154, a dot line D3 has been formed by the nozzle 155, a dot line D4 has been formed by the nozzle 156, and a dot line D5 has been formed by the nozzle 157.
The nozzles 153˜157 might not be equidistant in the X direction. In the example of FIG. 18, the nozzle 155 is slightly displaced toward the right. In this case, the dot line D3 is formed slightly displaced toward the right The dot line D2 and the dot line D3 barely overlap, and there is a large overlap of the dot line D3 and the dot line D4. In this case, ink density between the dot line D2 and the dot line D3 is much less than in other portions. The region in which the ink density is smaller extends continuously in the Y direction. Further, the ink density between the dot line D3 and the dot line D4 is much greater than in other portions. The region in which the ink density is greater extends continuously in the Y direction. When the region in which the ink density is smaller or greater extends continuously in the Y direction, a user can perceive a striped pattern that extends in the Y direction. Printing results are thus unsatisfactory.
The technique set forth in Japanese Patent Application Publication No. 2004/345167 will be described with reference to FIG. 19. An ink jet head 202 of an ink jet printer 201 has a plurality of nozzle units 203˜207. The nozzle unit 203 has a pair of nozzles 203a and 203b that are aligned in a direction (a Y direction) in which the ink jet head 202 moves with respect to a print medium 200. The other nozzle units 204˜207 each have a configuration similar to the configuration of the nozzle unit 203. That is, the nozzle units 204˜207 have nozzles 204a˜207a and nozzles 204b˜207b. The nozzles 203a˜207a and nozzles 203b˜207b can discharge the same color ink.
The nozzle unit 203 can form one dot on the print medium by discharging ink droplets from either of the nozzles 203a and 203b. The other nozzle units 204˜207 can also form one dot on the print medium by discharging ink droplets from either of the nozzles.
With the technique of FIG. 19, an external device (for example, a PC) connected with the ink jet printer 201 selects one nozzle at random from the nozzles of the nozzle unit which corresponds to the position at which the dot is to be formed.
For example, if the position at which a dot is to be formed is P11, one nozzle (the nozzle 203a or the nozzle 203b) is selected at random from the nozzle unit 203 that corresponds to P11. In the case where the external device has selected the nozzle 203a, the external device creates information including the combination of P11 and the nozzle 203a. 
As another example, if the position at which a dot is to be formed is P12, one nozzle is selected at random out of the nozzles 203a and 203b. In the case where the external device has selected the nozzle 203b, the external device creates information including the combination of P12 and the nozzle 203b. 
As another example, if the position at which a dot is to be formed is P21, one nozzle (the nozzle 204a or the nozzle 204b) is selected at random from the nozzle unit 204 that corresponds to P21. In the case where the external device has selected the nozzle 204b, the external device creates information including the combination of P21 and the nozzle 204b. 
The external device creates data that includes a plurality of combinations of position and nozzle. Below, this data will be termed print data. The external device outputs the print data to the ink jet printer 201. The ink jet printer 201 discharges ink from the nozzles based on the print data. For example, in the case where print data has been obtained having the combination of P11 and the nozzle 203a, the ink jet printer 201 discharges ink from the nozzle 203a toward P11. As another example, in the case where print data has been obtained having the combination of P12 and the nozzle 203b, the ink jet printer 201 discharges ink from the nozzle 203b toward P12. As another example, in the case where print data has been obtained having the combination of P21 and the nozzle 204b, the ink jet printer 201 discharges ink from the nozzle 204b toward P21.
In FIG. 19, hatching has been applied to the dots formed by the nozzles 203a˜207a Hatching has not been applied to the dots formed by the nozzles 203b˜207b. 
In the nozzle line D3 of FIG. 19, the dots formed by the nozzle 205a are displaced toward the right. The dots formed by the nozzle 205b are not displaced. The dots of the other nozzle lines D1, D2, D4, and D5 are also not displaced.
With this technique, if the nozzle 205a is not aligned equidistantly in the X direction, the dot line D3 will not be formed only by the nozzle 205a, but will instead be formed by both the nozzle 205a and the nozzle 205b. As a result, some dots in the dot line D3 are not displaced. With this technique, it may be possible to prevent in which the ink density is much greater or smaller from continuing across a wide range. Better printing results can be obtained with this technique than with the conventional technique described using FIG. 18.